1. Field of the Invention
The invention relates to a solid electrolytic capacitor having an electrically-conductive polymer as a solid electrolyte and a method for production thereof.
2. Description of the Related Art
In accordance with recent trend in reducing the size and weight of electronic equipment, there are increasing demands for a compact capacitor having large capacitance and exhibiting low impedance in a high frequency range. Therefore, the solid electrolytic capacitor having an electrically-conductive polymer as a solid electrolyte is further demanded reduction of ESR(Equivalent Series Resistance) as characteristic improvement in the high frequency range. Herein, examples of the electrically-conductive polymer include polymers produced by polymerizing a monomer such as pyrrole, thiophene, furan, or aniline.
Known methods for producing electrically-conductive polymer include the chemical polymerization method and the electrolytic polymerization method. In general, as compared with the chemical polymerization method, the electrolytic polymerization method can produce an electrically-conductive polymer film excellent in mechanical strength and electrical conductivity. However, when the electrically-conductive polymer produced by electrolytic polymerization is used to form a solid electrolyte layer, the surface of the solid electrolyte layer tends to be smooth, and adhesion between the solid electrolyte layer and a cathode lead layer formed thereon becomes poor, so that ESR can increase in the high frequency range.
On the other hand, known methods for reducing ESR include improving the adhesion between a solid electrolyte layer and a cathode lead layer. Known examples of such methods include a technique that includes enhancing the adhesion between a cathode lead layer and a solid electrolyte layer by depositing, on a capacitor element, an electrically-conductive powder for forming irregularities on the surface of the solid electrolyte layer (for example, Japanese Patent Application Laid-Open (JP-A) No. H07-94368) and a technique that includes forming an electrically-conductive polymer film by a chemical polymerization on an electrically-conductive polymer film formed by an electrolytic polymerization in the process of preparing a solid electrolyte layer so that good adhesion between the cathode lead layer and the solid electrolyte layer is provided (for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-133549).
However, the former technique (JP-A No. H07-94368) has a problem in which the number of the processes for depositing the electrically-conductive powder on the element increases, and the electrically-conductive powder makes a solid electrolyte layer thick so that the thickness of the capacitor increases.
The latter technique (JP-A No. 2000-133549) also has a problem in which since the formation of the electrically-conductive polymer film by the electrolytic polymerization is followed by the formation of the electrically-conductive polymer film by the chemical polymerization, the number of processes increases, and the production of the capacitor is not easy.